JPH07192302A - Optical head device having super high resolution - Google Patents

Abstract

PURPOSE:To form a main spot of a diffraction limit or below and make laser light stably follow a track without loss accompanying the situation in which three beams are used. CONSTITUTION:The laser light emitted by a laser light source 1 is guided to a collimator lens 2, a super high resolution modulator 3, a polarization beam splitter 4, and a lambda/4 plate 5 to form the main spot and side spots 8a and 8b on a recording medium 7 through an objective 6. This super high resolution modulator 3 is formed in a repetitive pattern of transmission areas and light shield areas composed of diffraction gratings, and consequently a track error signal is detected by forming the main spot 8 of the diffraction limit or below from the transmitted light and also using the side spots of low sidelobe intensity formed from diffracted light.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head device used in an optical recording / reproducing apparatus for reproducing information on a recording medium or for recording / reproducing or recording / reproducing / erasing information on a recording medium. Is.

[0002]

2. Description of the Related Art In optical recording, a device such as an optical disk for optically reproducing information on a concentric or spiral track formed on the disk has already been put into practical use.

In recent years, along with the demand for an increase in storage capacity, it is necessary to increase the recording density in such an apparatus, and for that reason, it is required to reduce the spot condensed on the recording medium. However, although the size of the spot is determined by the wavelength of the laser light source and the NA (numerical aperture) of the objective lens, there is a limit to shortening the wavelength of the semiconductor laser and increasing the NA of the objective lens. Therefore, in recent years, research on a super-resolution technique for obtaining a spot diameter equal to or smaller than a diffraction limit spot diameter determined by the wavelength of a laser light source and the NA of an objective lens has been advanced, and an example of applying the super-resolution technique to an optical head device has been made. Proposed (for example, JP-A-1
-315040, Japanese Patent Laid-Open No. 1-315041). In addition, as a region for partially modulating the light intensity of the laser light source, a ring-shaped light-shielding region is connected by using a repeating pattern in which the central portion is shielded, the peripheral portion is transmitted, and the peripheral portion is shielded. An optical head device (hereinafter referred to as a super-resolution optical head device) that obtains a super-resolution effect has been proposed (see Japanese Patent Laid-Open No. 2-91829).

The structure of the optical system of this type of basic super-resolution optical head device will be described with reference to FIG. Further, the shape of the spot formed on the recording medium will be described with reference to FIG.

In the super-resolution optical head device shown in FIG. 4, the laser light emitted from the laser light source 1 is collimated by the collimator lens 2 and enters the super-resolution modulator 40. The super-resolution modulator 40 has a light-shielding region at the center as shown in the super-resolution modulator shape 40a, and gives a super-resolution effect to a focused spot on the recording medium 7. Then, the laser beam that has passed through the super-resolution modulator 40 has a diffraction grating shape 41a.
The entire area as shown in FIG. 3B is converted into three beams by passing through the linear diffraction grating 41, and the polarization beam splitter 4, λ / 4
The light passes through the plate 5 and is condensed by the objective lens 6 to form a main spot 8 and side spots 8a and 8b on the recording medium 7.

The information signal light reflected from the recording medium 7 passes through the objective lens 6 and the λ / 4 plate 5, is totally reflected by the polarization beam splitter 4, and is incident on the beam splitter 9. At this time, the information signal light incident on the beam splitter 9 is split into two beams, and one beam is transmitted through the beam splitter 9 and focused by the refocusing lens 13 and then set at the reimaging position. The light passes through the pinhole 14 and is received by the photodetector 15. The other beam is reflected by the beam splitter 9 and is condensed by the re-condensing lens 10, and the cylindrical lens 11 causes astigmatism to be received by the photodetector 12.

As shown in FIG. 5, on the recording medium 7, a main spot 8 having a side lobe 32 around a main beam 31, a side spot 8a having a side lobe 34a around a main beam 33a, and a main beam 33
Side spot 8 with side lobe 34b around b
b is formed, and the side spots 8a and 8b are the recording medium 7
Irradiation is performed at a position rotated by a minute angle around the main spot 8 so as to be located on both sides of the upper track 45. Main spot 8 and side spots 8a, 8b
As shown in the cross section AA ′ in FIG. 5, the intensity distributions of No. 1 and No. 2 are different only in the absolute value of the intensity, and the intensity distributions are the same.

In this super-resolution optical head device, the main spot 8 focused on the recording medium 7 is moved to the predetermined track 4
As a method of tracking the signal of No. 5, a known three-beam method is used in which the track error signal is detected by detecting the difference between the light amounts of the side spots 8a and 8b received on the photodetector 12.

As a method of focusing the main spot 8 focused on the recording medium 7 by the objective lens 6, astigmatism is generated from the beam reflected by the recording medium 7 using the cylindrical lens 11. Photo detector 12
This is performed by a known astigmatism method that detects a focus error signal from the amount of astigmatism received above.

The information signal is transmitted through the central portion as shown in the pinhole shape 14a to the re-imaging position and uses the pinhole 14 which shields the outside thereof to remove the side lobe imaged at the re-imaging position. As a result, the side lobes 32 formed on the recording medium 7 are shielded, only the main beam 31 is guided to the photodetector 15, and the change in the amount of light received by the photodetector 15 is detected.

[0011]

However, in the conventional super-resolution optical head device, a side lobe is generated in the side spot for tracking the track of the recording medium. Therefore, the beam received by the photodetector that detects the track error signal is received as a mixture of the main beam of the side spot irradiated on the recording medium and the side lobes, and the track error signal affects the influence of adjacent tracks. Therefore, there is a drawback that track tracking becomes unstable due to the inclusion of noise components such as crosstalk noise.

Furthermore, in the case of a super-resolution optical head device that modulates the light intensity of laser light, the amount of light of the main spot focused on the recording medium decreases due to the loss of laser light due to the super-resolution modulator, which leads to track error. By combining the detection method with the three-beam method, the light amount of the main spot is further reduced. Therefore, there is a drawback that a high-power laser light source is required.

Therefore, an object of the present invention is to provide a super-resolution optical head device which enables stable track following by forming a side spot having a small side lobe intensity and which does not require a high-power laser light source due to a decrease in light quantity. That is.

[0014]

Therefore, in order to solve the above-mentioned problems, the present invention uses a ring-shaped region composed of a diffraction grating as a region for reducing the light intensity of a super-resolution modulator, The main spot below the diffraction limit obtained by the super-resolution effect and the side spot with a small side lobe intensity obtained by the diffraction grating are formed to enable stable track following.

[0015]

According to the present invention, the side spot for tracking the track of the recording medium is a side spot having a small sidelobe intensity, so that the problem of instability of the track following is solved. Furthermore, since the laser light that has been lost in the conventional super-resolution optical head device to reduce the light intensity can be effectively used, the problem of increasing the output of the laser light source is solved.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of one embodiment of the present invention will be described with reference to FIGS.

FIG. 1 is a block diagram showing an embodiment of an optical system of a super-resolution optical head device according to the present invention, FIG. 2 is a block diagram showing an embodiment of a super-resolution modulator according to the present invention, and FIG. 2 is a result of simulating the spot shape formed on the recording medium by the super-resolution optical head device of FIG.

First, the structure of the optical system of the super-resolution optical head device of the present invention will be described with reference to FIG. The laser light emitted from the laser light source 1 is collimated by the collimator lens 2 and passes through the super-resolution modulator 3 to form three beams.
The light passes through the polarization beam splitter 4 and the λ / 4 plate 5 and is condensed by the objective lens 6 to form a main spot 8 on the recording medium 7.
And side spots 8a and 8b are formed. The information signal light reflected from the recording medium 7 passes through the objective lens 6 and the λ / 4 plate 5, is totally reflected by the polarization beam splitter 4, and is incident on the beam splitter 9. At this time, the information signal light incident on the beam splitter 9 is split into two beams,
One of the beams passes through the beam splitter 9, is condensed by the recondensing lens 13, and then passes through the pinhole 14 provided at the reimaging position, and is received by the photodetector 15. The other beam is reflected by the beam splitter 9 and the refocusing lens 10
The cylindrical lens 11 causes astigmatism to be generated and the photodetector 12 receives the light.

Next, the relationship between the structure of the super-resolution modulator according to the present invention and the shape of the spot formed on the recording medium will be described with reference to FIGS.

As shown in FIG. 2, the super-resolution modulator shape 3a
In the case where the beam 20 collimated by the collimator lens 20 has a repeating pattern of the transmissive region 21 and the light-shielding region 22, the reduced main spot 8 is obtained on the recording medium 7 by the super-resolution effect.

The light-shielding region of the super-resolution modulator 3 is composed of a phase type diffraction grating 23. The grating spacing of the diffraction grating 23 is set so that the spacing between the focused spots of the 0th-order diffracted light and the 1st-order diffracted light on the recording medium 7 is about 20 μm, and the width of the convex portion and the concave portion of the grating is 1: 1 and the phase difference By setting π, the 0th-order diffracted light is not generated, and the transmittance of the light shielding region 22 becomes 0%.
Further, by setting the width and phase difference between the convex portion and the concave portion of the diffraction grating 23 and generating 0th-order diffracted light, the light-shielding region 2
It is possible to freely select the transmittance of 2.

For example, the laser light source has a wavelength of 680 nm, the emission angle is 8 ° × 21 °, the objective lens has a focal length of 4.1 mm,
NA 0.55, collimating lens focal length 25.0m
m, the radius of the central light-shielding region is 0.5 mm, the radius of the peripheral light-shielding region is 0.8 mm to 1.8 mm, the light-shielding region 22
On the recording medium 7 with the transmittance of 0% set to 0%, the main spot 8 obtained by the transmitted light 24 and the + 1st-order diffracted light 24a and the -1st-order diffracted light 24b are obtained ( side spots 8a as shown in b),
8b is formed. Main spot 8 is main beam 3
1 and side lobes 32, the main beam 31
Since the spot diameter (1 / e2) is 0.92 × 0.82 μm, the spot diameter reduction effect is about 80% with respect to the diffraction-limited spot diameter (1 / e2) of 1.10 × 1.01 μm. Is obtained. Also, the side spots 8a, 8b
Is similarly formed from the main beam 33 and the side lobe 34, the spot diameter (1 / e2) of the main beam 33 is 1.27 × 1.19 μm, and the intensity of the side lobe 34 is 4.1 of the intensity of the main beam 33. %.

Further, the side spots 8a and 8b are irradiated at a position rotated by a minute angle about the main spot 8 so as to be located on both sides of the track on the recording medium 7.

In this embodiment, a high-order side lobe can be generated to form a main spot having a small first-order side lobe intensity of 6.1%, and the side spot obtained by the diffraction grating is a side lobe intensity. Can be reduced to 4.1%.

That is, in the case of using the super-resolution modulator shown in the conventional example of FIG. 4 in which only the central portion is the light-shielding area having a radius of 1.4 mm, the main beam diameter of the main spot (1 / e
2) can be reduced to the same level as 0.89 × 0.81 μm, but the primary side lobe intensity of the main spot is as large as 16.6%. When a new diffraction grating is added to this super-resolution optical head, the side lobe intensity of the side spot is also 16.6%. Further, when the light-shielding region is formed of a diffraction grating, the main beam diameter (1 / e2) of the side spot is increased to 1.72 × 1.66 μm, which is not practical.

In the super-resolution optical head device of the present invention, as a method for causing the main spot 8 focused on the recording medium 7 to follow a predetermined track, the side spots 8a and 8b reflected from the recording medium 7 are detected by the photodetector 12. This is performed by a known three-beam method in which the above-mentioned light is received independently and the track error signal is detected by detecting the difference in the amount of received light.

As a method of focusing the main spot 8 focused on the recording medium 7 from the objective lens 6, astigmatism is generated from the beam reflected from the recording medium 7 by using the cylindrical lens 11. This is performed by a known astigmatism method in which the focus error signal is detected by detecting the amount of astigmatism.

The information signal is transmitted through only the central portion and the peripheral portion is shielded from light in the peripheral portion, as shown in a pinhole shape 14a, and the peripheral portion of the pinhole 14 is shielded from light by the side of the main spot 8. The lobe 32 is shielded, and only the main beam 31 of the main spot 8 is detected by the photodetector 15.
And is detected from the change in the amount of light received by the photodetector 15.

In the embodiment, the example of the infinite optical system using the collimator lens is described, but the present invention is also applicable to the finite optical system not using the collimator lens.

The present apparatus is also applicable to all super-resolution optical head devices used for read-only recording media, write-once recording media, or rewritable recording media.

In this embodiment, one method of detecting the focus error on the recording medium is shown, but any other known method can be applied.

[0032]

As described above, the present invention is obtained by the super-resolution effect by using the ring-shaped region constituted by the diffraction grating as the region for reducing the light intensity of the super-resolution modulator. Since a main spot below the diffraction limit and a side spot with a small side lobe intensity obtained by the diffraction grating can be formed, there is an effect that stable track following is possible. Furthermore, since there is no loss of laser light associated with the three-beam structure, the laser light source can have a low output.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of an optical system of a super-resolution optical head device according to the present invention.

FIG. 2 is a configuration diagram showing an embodiment of a super-resolution modulator according to the present invention.

FIG. 3 is a result of simulating a spot shape formed on a recording medium by the super-resolution optical head device of the present invention.

FIG. 4 is a configuration diagram of an optical system of a conventional super-resolution optical head device.

FIG. 5 is an explanatory diagram showing a spot shape formed on a conventional recording medium.

[Explanation of symbols]

 1 Laser Light Source 2 Collimating Lens 3 Super Resolution Modulator 3a Super Resolution Modulator Shape 4 Polarization Beam Splitter 5 λ / 4 Plate 6 Objective Lens 7 Recording Medium 8 Main Spot 8a Side Spot 8b Side Spot 9 Beam Splitter 10 Refocusing Lens 11 Cylindrical lens 12 Photodetector 13 Re-focusing lens 14 Pinhole 14a Pinhole shape 20 Beam 21 Transmission area 22 Light shielding area 23 Diffraction grating 24 Transmitted light 24a + 1st order diffracted light 24b -1st order diffracted light 31 Main beam 32 Sidelobe 33 , 33a, 33b Main beam 34, 34a, 34b Sidelobe 40 Super-resolution modulator 40a Super-resolution modulator shape 41 Diffraction grating 41a Diffraction grating shape 45 Track

Claims (1)

[Claims] 1. A super-resolution optical head device for forming a spot diameter below a diffraction limit on a recording medium by modulating a partial light intensity of light emitted from a laser light source, wherein the laser light source and an objective lens are combined. In between, a super-resolution modulator for modulating the light intensity distribution of the emitted light in a ring-shaped region is provided,
The ring-shaped region is formed by a diffraction grating, and a first spot reduced to a diffraction limit or less by the super-resolution modulator, and two or more first spots formed on the recording medium by the diffracted light of the diffraction grating. Two spots are formed, the first spot is used for recording, reproducing, and erasing information on the recording medium, and the second spot is used for following a recording track of the recording medium. Super resolution optical head device.